1. Field of the Invention
The invention relates to an electron source substrate having one or a plurality of electron-emitting devices and to an image-forming apparatus using the electron source substrate in which a plurality of electron-emitting devices are arranged in a matrix shape and connected by wirings.
2. Related Background Art
Hitherto, with respect to an electron source substrate in which an electron-emitting device comprising a pair of device electrodes and an electroconductive thin film which is formed over the device electrodes and has an electron-emitting region is formed on an insulative substrate, when the surface of the substrate is charged, electron-emitting characteristics of the electron-emitting device become unstable and a discharge deterioration of the electron-emitting device is caused. Therefore, there has been known a method whereby the surface of the substrate on which the electrodes and the electroconductive thin film have been formed is spray-coated with a coating liquid containing a component material of an antistatic film and baked, thereby forming the antistatic film (for example, refer to Japanese Patent Application Laid-Open Nos. H08-180801 and 2002-358874).
The device electrodes and the electroconductive thin film which construct the electron-emitting device are formed on the substrate on which the antistatic film is formed and, further, X-directional wirings and Y-directional wirings are formed on the electron source substrate which is used for an image-forming apparatus and in which a plurality of electron-emitting devices are matrix-driven. Therefore, such a situation that a thickness of antistatic film near the electron-emitting device increases due to a delicate balance of thicknesses of the device electrodes, the electroconductive thin film, the X-directional wirings, and the Y-directional wirings or the like and a sheet resistance decreases extremely is liable to occur. Particularly, when the antistatic film is spray-coated, distribution of the thickness of antistatic film is liable to occur due to conditions such as surface tension of the coating liquid, a contact angle of the substrate surface as a substratum film, and the like in addition to the above conditions. If the thickness of antistatic film is large and the sheet resistance decreases extremely as mentioned above, even at the time of a low voltage (for example, low voltage at which electron emission regarding the non-selection devices is not caused) in a non-driving mode, a micro current flows, so that there is an problem of an increase in electric power consumption. In the case where such an electron source substrate is used for the image-forming apparatus, a driver IC for driving of a capacity which is larger than an inherently necessary capacity by an amount of such a leakage current has to be used, resulting in an increase in costs.
Particularly, it has been found that in the antistatic film near the electron-emitting region, an influence of the increase in leakage current mentioned above is large. This point will be described with reference to FIG. 22.
In FIG. 22, reference numeral 1 denotes an insulative substrate; 2 and 3 a pair of device electrodes; 4 an electroconductive thin film formed over the device electrodes 2 and 3; 5 a gap serving as an electron-emitting region; and 6 an antistatic film. According to the studies of the present inventors et al., it has been found that even if the antistatic film 6 is formed as a high-resistance film, as shown by a path (current path) indicated by arrows in FIG. 22, a predetermined amount of current flowing through an area of the antistatic film 6 adjacent to the electroconductive thin film 4 exists and such a current amount largely influences a value of the leakage current. Although a detailed phenomenon is obscure, according to the consideration of the present inventors et al., it has been found that since the portion of the gap 5 of the electroconductive thin film 4 has an extremely high resistance, a voltage across the device electrodes 2 and 3 through the electroconductive thin film 4 is concentrated on the gap 5 (the electroconductive thin film 4 of the left side in the case where the gap 5 is used as a boundary has almost the same electric potential as that of the device electrode 2, the electroconductive thin film 4 of the right side has almost the same electric potential as that of the device electrode 3, and the gap 5 becomes the actual voltage applying portion), the antistatic film 6 of the area which is come into contact with the electroconductive thin film 4 near the gap 5 becomes a path (current path) whose resistance is lower than those of the other portions, and the leakage current is concentrated.
Since a similar current path also exists between the pair of device electrodes although an extent of the current path is smaller than that of the portion near the electron-emitting region, a measure against the leakage current is also necessary between the pair of device electrodes 2 and 3.